我试图在C中实现一个简单的Unix shell。但是,我无法使用sigaction实现后台进程功能。我的代码结构如下:

int main() {
struct sigaction act;
act.sa_handler = handler;

sigaction(SIGCHLD, &act, 0);

while(1) {
    parseCommand();
    execCommand();
}
}

另外,在我的execCommand函数中,结构如下:
if (fork()) {
    if (!isBackground) {
        wait(&child_status);
    }
    else
    printf("Background process\n");
}
else
    ... // Command execution...

我的处理者如下:
void handler(int s) {
while (waitpid(-1, NULL, WNOHANG) > 0) {
}
printf("OK.\n");
}

但这种结构损害了我的整个实现。甚至前台进程也不起作用。当我执行前台命令时,它会给出:
Segmentation fault (core dumped)

当我执行后台命令时,它会给出:
Bus error (core dumped)

那么,我怎样才能解决这个问题呢?
提前谢谢。。。
编辑:当我用valgrind调试代码时,它给出了以下信息:
==6768== Memcheck, a memory error detector
==6768== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==6768== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==6768== Command: ./shell
==6768==
==6768== Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)
==6768==    at 0x4E6F5AE: __libc_sigaction (sigaction.c:62)
==6768==    by 0x400E90: main (in /home/enes/Desktop/Shell/shell)
==6768==  Address 0xffefff908 is on thread 1's stack
==6768==
==6768== Syscall param rt_sigaction(act->sa_flags) points to uninitialised byte(s)
==6768==    at 0x4E6F5AE: __libc_sigaction (sigaction.c:62)
==6768==    by 0x400E90: main (in /home/enes/Desktop/Shell/shell)
==6768==  Address 0xffefff8f8 is on thread 1's stack
==6768==
> ls
==6768== Use of uninitialised value of size 8
==6768==    at 0x4EA8C80: _IO_getline_info (iogetline.c:77)
==6768==    by 0x4EA7B7C: fgets (iofgets.c:53)
==6768==    by 0x400EC9: main (in /home/enes/Desktop/Shell/shell)
==6768==
==6768== Invalid write of size 1
==6768==    at 0x4EA8C80: _IO_getline_info (iogetline.c:77)
==6768==    by 0x4EA7B7C: fgets (iofgets.c:53)
==6768==    by 0x400EC9: main (in /home/enes/Desktop/Shell/shell)
==6768==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==6768==
==6768==
==6768== Process terminating with default action of signal 11 (SIGSEGV)
==6768==  Access not within mapped region at address 0x0
==6768==    at 0x4EA8C80: _IO_getline_info (iogetline.c:77)
==6768==    by 0x4EA7B7C: fgets (iofgets.c:53)
==6768==    by 0x400EC9: main (in /home/enes/Desktop/Shell/shell)
==6768==  If you believe this happened as a result of a stack
==6768==  overflow in your program's main thread (unlikely but
==6768==  possible), you can try to increase the size of the
==6768==  main thread stack using the --main-stacksize= flag.
==6768==  The main thread stack size used in this run was 8388608.
==6768==
==6768== HEAP SUMMARY:
==6768==     in use at exit: 0 bytes in 0 blocks
==6768==   total heap usage: 2 allocs, 2 frees, 2,048 bytes allocated
==6768==
==6768== All heap blocks were freed -- no leaks are possible
==6768==
==6768== For counts of detected and suppressed errors, rerun with: -v
==6768== Use --track-origins=yes to see where uninitialised values come from
==6768== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)
Segmentation fault (core dumped)

最佳答案

Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)

您需要完全初始化sigaction结构,否则它包含不确定的值。阅读这些(可能通过sigaction()函数调用)很可能会导致Undefined Behaviour,从中可以发生任何事情。
修正这个变化
struct sigaction act;

成为
struct sigaction act = {0};

可能不是你面临的问题,但仍然:
从信号处理程序调用printf()-函数系列的任何成员都不保存。
更重要的是:可以从信号处理程序安全调用的函数集(非常)有限。有关完整列表,请参见chapter 2.4.3 on this page
修正这个变化
printf("OK.\n");

成为
write(STDERR_FILENO, "OK.\n", sizeof "OK.\n" -1);

或者类似的。根据上面链接的列表,将write()保存为从信号处理程序调用。
==6768== Use of uninitialised value of size 8
==6768==    at 0x4EA8C80: _IO_getline_info (iogetline.c:77)
==6768==    by 0x4EA7B7C: fgets (iofgets.c:53)

调用fgets()实际上有什么问题我们无法判断,因为您没有向我们显示相关代码。

10-07 19:09
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